Secretary of State Rex Tillerson’s concerns that Russians are already working to influence the 2018 U.S. midterm elections were echoed Wednesday by Sen. Mark Warner (D-Va.), the ranking member of the Senate Intelligence Committee. Warner challenged Tillerson’s assertion that such hacking will be “very difficult to preempt,” and criticized President Donald Trump for not acknowledging the ongoing threat.

“While we’ve got individuals within the Trump administration acknowledging this threat, we don’t have a voice in the White House who seems to acknowledge the reality of this threat,” Warner said in an interview.

Trump has long dismissed military and congressional investigations that show online efforts by Russians influenced the 2016 election via the spreading of false or confidential information. On Saturday, Trump again called the Russian investigation a “witch hunt” and on Monday he accused Warner of being “one of the biggest liars and leakers in Washington.”

Warner, who has co-led the Senate inquiry into Russian influence on the election, has been an outspoken critic of Trump’s attacks on special counsel Robert Mueller, who was appointed in 2017 to investigate alleged Trump-Russia ties.

The U.S. could come up with a comprehensive offensive cyber warfare plan to counter online attacks from countries like Russia, Warner said as an example of how the U.S. could counter election hacks. The federal government could also assist states in improving election security, he added.

Trump’s dismissal of the Russian threat leads to ineffectiveness in repealing current and future digital attacks from the Kremlin, Warner said.

“The Russians got a great return on investment on their elections in 2016. For the cost of one new airplane, they managed to sow dissension in our country,” Warner said. “I think we are doing some good things, but what we lack — when the president of the United States doesn’t acknowledge this threat, we lack a whole of government approach.”